ASR slowly moves into Mississippi’s Delta

Mississippi’s Delta was Asian soybean rust-free until an Aug. 12 find in a Stoneville sentinel plot. Less than a week later, the disease had also been discovered in Sunflower and Hancock counties.

With the state’s soybean crop rapidly maturing and a forecast for additional hot, dry weather, there’s little worry ASR could become a major issue in 2007.

“We’ve searched Delta-wide for ASR — with a lot of people who know what it looks like — and we’ve found no more,” says Billy Moore, Mississippi Extension plant pathologist. “Outside the Delta, the Hancock County find was two small areas in a 10-acre field.”

The optimum temperature for ASR development is 67 to 77 degrees. The only time that temperature is being reached is at night.

“But in addition to that temperature range, this rust needs at least one and a half hours of free moisture for spores to germinate. It takes an additional six hours for infection of the plant to take place.”

Mississippi is currently so hot and dry, even if rust spores land in a field “they won’t do anything. I’m quite certain of that.”

The only soybeans Moore and colleagues are concerned about are those “exposed to a massive rain — 6 to 12 inches of rain in the Delta over 10 days — about a month ago. That’s apparently when this infection took place. If weather changes, the wheat-beans need to be watched closely.”

Moore says the hottest temperature he’s recorded this summer was 105 on Aug. 16. “That was in the field. The truck thermometer said it was hotter than that. At these temperatures, rust won’t get revved up.”

Speaking on Aug. 17, Moore said recent ASR development has only occurred in the immediate vicinity of the initial infection. For example, “look at the Sunflower County ASR discovery by Tom Allen (Mississippi State University Extension plant pathologist for the Delta). He looked down, saw rust, picked some leaves and then couldn’t find any more. That’s the kind of situation we have.

“And that tells us there’s probably spotty infections in scattered locations around the Delta area. But by (Aug. 23), 80 percent of our soybean crop will be at R-6 and out of danger.”

Problems in the state’s soybeans are due to dry weather, not ASR. “I was in a Magnolia-area dryland field two days ago. They’d called to say, ‘The whole field has turned silver.’ Turned out this field was reacting to dry weather. Three weeks ago, it had about 2.5 inches of rainfall and the producer had at least a 60-bushel yield potential. Pods were all over the plants.”

But then the rain shut off and heat sapped the dryland crop’s moisture. “Between charcoal rot and no rainfall, the plants silvered up. The drought reduced the size of the seeds considerably and the plants won’t fully recover. That’ll cost the grower a big chunk of yield — at least 15 bushels — but he’ll still be happy with that being dryland.

“If this drought continues, we’ll see more and more fields shutting down like that — particularly in lighter soils.”